AST value in athletes: muscle or liver?

The assay counts how much AST is circulating in your blood, in u/l. For an athlete the question is simple. How many cells broke recently? And which tissue did they sit in?

In trained people, skeletal muscle is often the largest source. Eccentric loading does this most strongly. That is the braking half of a movement. Think of the descent of a squat. Or the landing in a jump. Or running downhill. Your soreness comes from there too.

The AST enzyme also comes from heart muscle, the kidneys, brain tissue and red blood cells. The moment liver cells are damaged, it leaks out there just as readily. The assay cannot separate those sources. You make that distinction with the three values beside it.

Creatine kinase, see CK, comes almost exclusively from muscle. That is your clearest signal. ALT is the liver-specific counterpart. If your AST is higher than ALT, muscle is the likelier source. Gamma-GT notably does not move with muscle damage. Together with alkaline phosphatase, that is your steadiest anchor.

That fourth value is the most underrated instrument you have. Are AST and ALT both raised while gamma-GT stays normal? Then a liver cause is less likely than the alarm suggests.

For anyone training hard, the question is rarely whether AST is raised. The question is: raised relative to what. Your baseline sits higher than that of someone who does not train. You carry more muscle, and it is damaged more often.

The sharpest research on this had untrained men do one heavy weightlifting session. Their transaminases climbed to values resembling liver disease. Seven days later they were still elevated. Gamma-GT and bilirubin did not move. Seven days is far longer than the soreness lasts. That is exactly why people are caught out.

A raised AST has several different causes. In your case training comes first. But heavy alcohol use, fatty liver disease and some medicines belong there too. Certain medicines only lift the result after weeks.

Look at the magnitude as well. A mildly raised value fits muscle or alcohol. A markedly raised value is more than ten times the upper limit. That almost always indicates acute viral hepatitis. Call your GP in that case. In conditions of the liver such as hepatitis the ratio tips. ALT then rises above AST.

The interpretation grid below is the heart of this page.

Are AST and ALT both raised after a heavy week? Then the first question is not which medicine you take. The first question is what CK and gamma-GT are doing.

A warning, and it is not theoretical. With extreme exertion, heat or dehydration, muscle tissue can break down massively. An unusually large training stimulus does the same. CK then climbs into the thousands of u/l. The muscles swell and hurt severely. The urine turns dark like cola. The kidneys are at risk. This is emergency care. Do not order a blood test and do not wait for a result.

When you draw matters more here than any other preparation.

Draw during a deload week. That is when your muscles are least busy repairing. It is therefore the moment a raised AST says something about your liver. Draw mid-way through a heavy block and you mostly measure your training load.

Leave at least 48 to 72 hours after the last heavy session. The same holds after an unaccustomed one. After a first exposure to a new movement, waiting longer is wise. That applies after a competition or an eccentric programme too. The effect can persist for a week.

Request CK alongside it. Without CK, a raised AST is a puzzle. With CK included it is usually clear at a glance. Ask when AST should be measured again as well. A repeat often says more than the first result.

Measure under comparable conditions. Your own series over time says more than a single value. Draw at the same point in your block each time. Then you compare like with like.

Does AST stay raised even though you took the rest? Is CK normal alongside it? Then discuss that result with your GP.